"Rock of the Month # 266, posted for August 2023" ---

The Millstone Grit

The Millstone Grit is especially well-developed in the Peak District but also occurs along the Pennine chain and adjacent areas of central and northern England. The coarse sandstones (aka grit or gritstone) are associated with local developments of conglomerates and shales. They are attributed to deposition in a deltaic environment, in contrast to the onshore swamps of the younger Coal Measures. Cross-bedding, seen in each photo but especially in Figure 3, is a characteristic sedimentary structure of these rocks. They are very tough, composed principally of hard quartz, though even these rocks are subject to slow erosion as individual sand grains slough off under stresses of wind, water or human activity. Note that cross-bedding may indicate either aqueous (e.g., deltaic, continental margin) or subaerial (eolian, desert) environments, the distinction of these possibilities requiring careful fieldwork (Selley, 1970, pp.12-13, 52-56, 78-87). A local case study is the Kinderscout Grit sequence (ibid., pp.81-87). The Grit, local representative of the Millstone Grit, comprises over 400 feet of strata, mainly coarse, pebbly arkosic sandstones, the base displaying huge channels cut into underlying shales, often over 100 feet deep and almost 1,000 feet wide.

Beyond the Peak, the Grit also outcrops or subcrops in parts of Yorkshire and Lancashire, Staffordshire and Shropshire (see Edwards and Trotter, 1954, pp.32-41; Hains and Horton, 1969, pp.34-39) and Leicestershire (Ford, 1972). The Grit becomes thinner and more shaley north from the Peak District, across the Askrigg Block (Trueman, 1971) towards the Scottish border (Taylor et al., 1971, pp.54-59). But the Grit or its equivalents are also more widely distributed (Jukes-Browne, 1886; Geological Survey, 1957). Thus the Grit is found also in: the Bristol area and Mendip Hills (Welch, 1933; Kellaway and Welsh, 1993); south Wales (Neville George, 1970; Bassett and Bassett, 1971; Carr, 2001); southwest Wales (IGS, 1960, 1962) and Ireland (Praeger, 1937). An equivalent is the Limestone Coal Formation and associated strata of the Clackmannan Group in the Glasgow area. These are a mixture of marine, deltaic, fluvial and coal-swamp strata that succeeded the lavas and volcaniclastic rocks of the Clyde Volcanic Plateau (Allison and Webster, 2017; Webster, 2020).

The Millstone Grit is broadly conflated with a time division, the Namurian stage, third of six subdivisions of the European Carboniferous, and basal stage of the Upper Carboniferous, circa 326 to 313 Ma. It is named for a city and region in Belgium where strata of this age were described. The tough, abrasive varieties of the Grit have long been prized for their excellence as millstones (Moore, 1983). A scattering of millstones, in varied degrees of completion, lie abandoned around the nearby Toad's Mouth rock. Stratigraphically beneath the Millstone Grit is the Carboniferous Limestone, which locally hosts base-metal deposits (lead) as well as nearby concentrations of a purple, banded variant of fluorite (fluorspar), known as Blue John, sought after since Roman times (Ford, 1996; see also Blue John Fluorite on these pages).

Fossils are generally poorly preserved in sandstones, especially when viewed in comparison to limestones, where precursor calcareous muds enveloped and swaddled remains as they were buried and mineralized. Yet fossils do occur in the Millstone Grit, including cephalopods (goniatites such as Reticuloceras, Black, 1972). Marine fossils include genera such as Goniatites and the bivalve Posidonia (Wells and Kirkaldy, 1966, pp.233-247). The fauna is in contrast to the later non-marine bivalves of the younger Coal Measures.

Fig. 3 : A third outcrop, a rock formation displaying cross bedding in sandstone, a landmark known as Mother Cap.